1. Field of the Invention
The present invention relates to a filter circuit having plural passbands.
2. Related Art of the Invention
The surface acoustic wave filter has come to be used in various appliances and, in particular, mobile communication devices owing to its features of small size and high performance. Characteristics of filters have been widely investigated, but mainly filters having a single passband have been studied, and little has been studied about filters having multiple passbands.
For example, what is disclosed in Japanese Laid-open Patent No. 5-167388 is a branching filter making use of surface acoustic waves. Japanese Laid-open Patent No. 9-98046 discloses a branching filter in which a circuit connecting a low pass filter is explained in the connection part of each filter. In Japanese Laid-open Patent No. 9-121138, similarly, a circuit used in a branching filter is described.
Recently, owing to advancement in mobile communications, the service frequency band is raised to the region of quasi-microwaves. As mobile communication is further distributed, the service frequencies tend to extend.
However, the use of the conventional system cannot be stopped, and therefore the coexistence of two systems is required. For this purpose, for transmission and reception, at least two or more frequencies must be used. In this case, a filter, reception amplifier and transmission amplifier are required for each frequency. That is, two sets of each are needed in this case, and the circuit scale is thereby increased.
As a circuit corresponding to both frequencies, for example, if the reception amplifier is used commonly, separate filters are needed for individual frequencies. In such a case, with changing over these filters by a switch using a semiconductor element, either appropriate filter can be used.
Also in this case, a power source circuit is needed for the semiconductor element, and the circuit scale is increased, too.
In the method explained in Japanese Laid-open Patent No. 5-167388, at a one-sided filter, an inductor or phase rotation line is inserted between the signal line and the ground, or a capacitor is inserted in series into the one-sided filter. In Japanese Laid-open Patent No. 9-98046, a circuit connecting a low pass filter in the connection part of each filter is disclosed. In Japanese Laid-open Patent No. 9-121138, a branching filter adding a phase adjusting circuit is disclosed.
In any case, however, since a circuit element lower in Q than the surface acoustic wave resonator is used, it finally leads to increase of insertion loss of the filter. Incidentally, the transmission line used as the phase adjusting circuit is extended in length depending on the frequency, and the merit of downsizing is decreased.
Needless to say, the above characteristic required in the branching filter is, in short, how to connect both filters without mutual interference of the characteristics of the individual filters. In such a branching filter, incidentally, taking note of one arbitrary filter out of the two filters to be connected, at the passing frequency of that filter, the impedance of the other filter is infinite. Thus conventional filter investigation is made from only one viewpoint of how to design so that the impedance of the other filter at the passing frequency may not contain the inductance components or capacity components.
However, the inventor of the present application, not sticking to the conventional fixed idea, discovered that the number of elements for matching can be decreased by designing from a different point of view.
That is, the new point of view of the inventor of the present application is to connect the circuit by allowing it to contain the inductance components or capacitive components in the impedance of one filter at the passing frequency of this filter. At this time, for this filter, at its passing frequency, the impedance of the other filter is a certain large value, but it is not infinite as in the case of the prior art method. Accordingly, regarding the other filter as a circuit element having the above impedance connected to one filter, a matching element is connected to the other filter, so that the effect of this circuit element can be substantially ignored. By adjusting the value of this matching element, the inductance components or capacitive components considered to be effects of the circuit element can be decreased or removed. Hence, while maintaining the same performance as in the prior art, the number of matching elements can be curtailed consequently. The inventor of the present applicant also discovered that it is sometimes not necessary to connect the matching element to the other filter as mentioned above, if the value of the component is an ignorable level, after connecting by allowing to contain the inductance components or capacity components.
In consideration of such problems in connection between conventional filters, it is an object of the invention to realize a filter circuit having plural passbands by the constitution further curtailing the number of matching elements.